Fluid controlling

ABSTRACT

A fluid control circuit comprising a controller including a control element movable between first and second positions and operative for generating an output fluid signal dependent on the position of the control element, and first and second sensors arranged for sensing a pilot pressure, one of the sensors being operable for causing the control element to assume its second position when the sensed pilot pressure exceeds a predetermined upper level, and the other of the sensors being operable for causing the control element to assume its first position when the pilot pressure drops below a predetermined lower level.

United States Patent Denker Jan. 29, 1974 1 FLUID CONTROLLING 3,587,6196/1971 Krechel 137/116 2,375,411 5/1945 Grant 137/116 [75] inventor:James Denker, scmlate Mass' 2,507,384 5/1950 Schneck 137/116 [73]Assignee: Nutron Corporation, Hingham,

MaSS- Primary Examiner-William R. Cline [22] Filed: Aug. 18, 1972 211Appl. No.: 281,966 [57] ABSTRACT Related s Application Data A fluidcontrol circuit comprising a controller includ- [63] continuatiommpan ofSer No. 230,561 March 1 ing a control element movable between first andseend positions and operative for generating an output fluid signaldependent on the position of the control 52 US. Cl 137/115, 137/116137/119 element, and first and sewnd Sensors arranged 137/488 sensing apilot pressure, one of the sensors being oper- 51 Int. Cl. Gd 16/10 ablefor causing the assume its [58] Field of Search 137/1 l 16 1 19 488 4910nd position when the sensed pilot pressure exceeds a 137/492predetermined upper level, and the other of the sensors being operablefor causing the control element to [56] References Cited assume itsfirst position when the pilot pressure drops UNXTED STATES PATENTS belowa predetermined lower level.

3,593,741 7/1971 Odenthal et a1. 137/491 X 18 Claims, 2 Drawing FiguresI16 I34 I28 118 I66 143 N4 120 I is; 22/ 21') c 1o 94 n FLUIDCONTROLLING This invention relates to fluid controls. This applicationis a continuation-in-part of my copending U.S. Pat. application Ser. No.230,561, filed Mar. 1, 1972.

It is a principal object of the present invention to provide a simpleand inexpensive fluid logic circuit that will discriminate betweenrising and falling pressure. Other objects include providing a modularunloading valve, preferably incorporating such circuitry, in which theunloading and reloading points are adjustable.

The invention features a fluid control circuit comprising a controllerincluding a control element movable between first and second positionsand operative for generating an output fluid signal dependent on thecontrol element position, and first and second sensors arranged forsensing a pilot pressure, one of the sensors being operable for causingthe control element to assume its second position when the pilotpressure exceeds a predetermined upper level, and the other of thesensors being operable for causing the control element to assume itsfirst position when the pilot pressure drops below a predetermined lowerlevel.

In preferred embodiments comprising a unloading valve including aplurality of interconnected valving modules and in which one of thesensors causes movement of the control element only in response torising pressure and the other causes movement only in response tofalling pressure, there is featured sensors comprising substantiallyidentical modules each including an adjustable relief valve, a bypassflow module including a poppet spool for regulating flow between a fluidsource and load, and a control module including the control elementconnected to the sensor and bypass modules for applying the fluidsignals to an end of the spool.

Other objects, features, and advantages will appear from the followingdetailed description of a preferred embodiment of the invention, takentogether with the attached drawings in which:

FIG. 1 is a schematic view of a fluid circuit including a plan sectionalview of an unloading valve embodying the present invention; and,

FIG. 2 is a plan sectional view of the valve of FIG. 1 taken at 2-2 ofFIG. 1.

Referring now to the drawings, there is shown an unloading valve,generally designated 10, whose main inlet 12 is connected by fluid line14 to a constant flow, fixed displacement pump 16, whose main outlet 18is connected by a fluid line 20 to a load 22, and whose drain outlet 24is connected to a tank or reservoir 26. An accumulator 28 is connectedto line 20 intermediate valve 10 and load 22.

As shown most clearly in FIG. 1, unloading valve 10 comprises five valvemodules, designated 30, 32, 34, 36, and 38 respectively. Modules 30, 32,and 34 are stacked one on top of the other, modules 36, 38 are attachedto opposite sides of module 34. The various modules are held together byhex-head bolts and the interfaces are sealed by O-rings.

Module 30 comprises a rectangular valve housing 40 in which there areprovided four drilled bores or conduits, each extending inwardly from aside wall of housing 40. Bore 42 defines a valving chamber and extendsinwardly from housing end wall 44. Bores 46, 48 extend through housing40, from bottom wall 50 to top wall 52, axially intersecting bore 42.Bore 54 includes two intersecting relatively perpendicular drilledportions and extends from top wall 52 to end wall 56.

The portion of valve chamber bore 42 extending from bore 48 toward bore46 is of smaller diameter than the rest of bore 42, thereby providing anannular step 58 in bore 42 on the side of bore 46 toward bore 48. Acylindrical check valve 60, having an interior bore 62 and closed at oneend by a nose portion 64 having a chamfered annular rim 66, isslip-fitted within the larger diameter portion of bore 42 with the rim66 of nose portion 64 adjacent and in position for seating over step 58.When the rim is seated on step 58, nose portion 64 closes bore 42 toprevent flow between bores 46 and 48. Check valve 60 also includes adamping orifice 67 extending radially through the side wall thereof frombore 62.

The end of bore 42 adjacent housing end wall 44 is closed by a threadedplug 68 having a cylindrical recess 70 of diameter equal to that ofcheck valve bore 62 in its inner end. A helical compression spring 72 ismounted with its opposite ends within bore 62 of check valve 60 andrecess 70 of plug 68, and urges check valve 60 towards its closedposition.

Module 32 comprises a rectangular housing 74 including four drilledconduits extending inwardly from the side walls thereof. Conduits 76 and78 extend between top surface 80 and bottom surface 82, and communicateand are in coaxial alignment with, respectively, conduits 46 and 54(upper portion) of module 30. Conduit 84 defines a valving chamber andextends inwardly from end surface 86, axially intersecting conduit 78,approximately two-thirds the length of housing 74. Conduit 88 extendsfrom bottom surface 82, intermediate conduits 76 and 78, to and axiallyintersects an intermediate portion of valving chamber conduit 84.Conduit 90 extends from the top surface 80 of housing 74 and axiallyintersects valving chamber conduit 84 adjacent the interior end thereof.

The exterior end of chamber conduit 84, adjacent end wall 86, is closedby a threaded plug 92 having a cylindrical recess 94 in the inner endthereof. The portion of conduit 84 extending from end wall 86 toslightly beyond conduit 78 is of larger diameter than the conduitportion extending coaxially therebeyond, providing an annular step 96closely adjacent conduit 78 on the side thereof toward conduits 88 and90.

A poppet spool 98 having a cylindrical bore 100 extending axiallytherethrough is slip-fitted within conduit 84. One end portion 102 ofpoppet spool 98 is closely slip-fitted within the smaller diameterportion of bore 84 intermediate intersecting bores 88 and 90. The otherend portion 104 of poppet spool 98 is slip-fitted within the largerdiameter portion of bore 84 closely adjacent annular step 96 andincludes on the side thereof facing end portion 102 a conical, generallyaxially facing surface 106 arranged for seating over step 96. Bore 100extending coaxially through spool 98 includes a small orifice 108 atspool end 102, and an increased diameter portion 1 10 defining anaxially facing seat at spool end 104. A helical compression spring 1 12is mounted with its opposite ends within recess 94 of plug 92 and boreportion 110 of spool 98, and urges spool surface 106 against annularseat 96.

The rectangular housing 114 of module 34 includes a valving chamber bore116 extending inwardly from end surface 118, bores 120 and 122 extendingfrom bottom surface 124 to and axially intersecting valving chamber bore116; bore 126 extending from bottom surface 124 to midway the height ofmodule 34; bore 128 extending between side surfaces 130, 132 and axiallyintersecting bore 126; bore 134 extending from side surface 130 to andaxially intersecting chamber bore 116 at the inner end thereof; bore 136extending from side surface 132 and axially intersecting chamber bore116 adjacent bore 122; and tapped gage port bore 138 extending from endsurface 140 to the intersection of bores 128 and 126. As shown, bores120, 122, and 126 communicate, respectively, with bores 78, 90 and 76 ofmodule 32. Bores 120, 124 are coaxially aligned with bores 7 8, 76respectively. An elongated recess 142 is provided in side 124 of housing1 14 overlying the end of conduit 122 and arranged also to overlie theend of conduit 98.

The major portion of valve chamber bore 116 is of slightly greaterdiameter than that closely adjacent and intersecting bore 134, providingan annular step 141. An annular valve seat 143 having a drilled orifice144 extending therethrough coaxially with chamber bore 1 16 is seated onstep 141, intermediate bores 136, 134. The end of chamber bore 1 16adjacent end wall 118 is closed by a plug 145 having a coaxiallycylindrical recess 146 in the inner end thereof.

A stepped-cylindrical poppet piston 148 having a smaller diameterportion 150 and larger diameter portion 152 joined by an axially facingsurface 153 is closely slip-fitted within bore 116 with the smallerdiameter portion 150 thereof adjacent valve seat 143 and the largerdiameter portion 152 thereof intermediate bores 120, 122. A cylindricalbore 154 of diameter equal to that of orifice 144 extends coaxiallythrough piston 148 and a piston nose 156 is fitted in the end of smallerdiameter portion 150 in position for overlying and closing orifice 144when piston 148 is seated against valve seat 142. A damping orifice 158extends through nose 156. A cylindrical balance pin 160 one end of whichis tightly slip-fitted within piston bore 154, extends axially ofchamber bore 116 from within piston 148 to within recess 136 of plug145, where it is fixed by a transverse pin 162. As illustrated, thelength and position of pin 160 are such that an internal chamber 164 isprovided within piston bore 154 between nose 156 and the adjacent end ofpin 160. A helical compression spring 166 bears against the adjacentends of plug 145 and piston major diameter portion 152, urging nose 156against seat 142.

As previously mentioned, modules 36 and 38 are identical. Accordingly,only module 38 will be described herein in detail. In the drawings, eachcorresponding part of module 36 is identified by the same referencenumber used in describing module 38, though with a differentiating addedthereto.

Module 38 includes a rectangular housing 170 having main bore 172extending longitudinally from end wall 174 substantially three-fourthsthe length thereof, and two transverse bores 176, 178 extending fromside wall 180 and axially intersecting main bore 172. Transverse bore176 is coaxial with and communicates with bore 128 of module 34.Communication between transverse bore 178 and bore 136 of module 34 isprovided by an elongated recess 182 in side wall 180 overlying the endof bore 178 and arranged to overlie also the end of bore 136.

A valve, generally designated 188, is provided in main bore 172. Asshown, valve 188 includes a valve seat 186 mounted intermediate bores176, 178 with the minor diameter portion thereof adjacent bore 178, apoppet piston 190 slidably mounted in main bore 172 and having a conicalhead 192 adjacent and facing valve seat 186, a threaded axiallyadjustable body member 194 extending coaxially through a retainingsleeve 184 in the end of bore 172 adjacent end of bore 172 adjacent endwall 174, and a helical compression spring 196 between body member 194and poppet piston 190 urging conical head 192 against valve seat 186. Alocking screw 198 is provided for setting the position of body member194 and thus the force with which poppet piston 190 is forced againstseat 186.

With reference to module 36, it will be seen that its bore 176a and 178aare coaxially aligned with and in communication with bores 128, 134,respectively, of module 34.

In operation, fluid from pump 16 passes into module 30, opens checkvalve 60, and flows through line 20 to load 22 and accumulator 28. Thefluid at the same pressure as that on load 22 and accumulator 28,hereafter referred to as load pressure, P also passes through conduit 46of module 30, conduit 76 of module 32, conduits 126 and 128 of module34, and conduits 176 and 176a of modules 38 and 36 to act on poppetpistons 190, 190a.

Pressure relief valve 188 of module 38 acts as an unloading relief, andis set to open at a higher level than pressure relief valve 188a ofmodule 36, which acts as a reloading relief.

As load pressure rises, it first reaches the setting of relief valve188a. As the pressure continues to rise, poppet piston 190a is forcedoff seat 186a, admitting fluid into conduits 178a and 134 where it actson the nose of poppet piston 148 and maintains a pressure that is equalto the load pressure, P less the setpoint of relief valve l88a. If, forexample, the relief valve setpoint is 500 psi, the pressure in conduits178a and 134 will remain at zero until the load pressure reaches 500psi, and will thereafter be equal to P 500 psi. It should be noted, asexplained in detail in my copending US. Pat. application Ser. No.230,561, that so long as piston 148 is seated on seat 141 there is nonet force across the piston nose and the piston position is not affectedby fluid pressure in conduits 178a, 134.

Pressure relief valve 188 in module 38 remains closed until P reachesits set point, which for example may be 985 psi. When P exceeds this setpoint, valve 188 opens, passing flow into conduits 178 and 136 at apressure of P 985 psi. The fluid from conduit 136 acts on axial surface153 of poppet piston 148 and, when the fluid pressure is sufficient toovercome the force of spring 166, for example 15 psi, lifts piston 148off seat 141. Fluid from conduit 134, at pressure 1 1-500, or some 485psi greater than that in conduit 136, now flows through orifice 144 intobore 116, and then through bores 122 and to act on smaller diameter end102 of poppet spool 98 of module 32. The force exerted by spring 1 12 onspool 98 is quite small and will be overcome by about 10 psi force. Thepressure of fluid from conduit 90 drives the spool to the left (as seenin FIG. 1), lifting spool sealing surface 106 off seat 96 and divertingpump flow from source 16 and inlet 12 through conduits 48, 84, 78 and 54to reservoir 26.

This flow diversion drops the pressure in bore 48 to atmosphericpressure, far below P and causes check valve 60 to close. Load 22 is nowdriven by fluid in accumulator 28, and the load pressure continuouslydrops. When P which as previously noted continues to act on pressurerelief valves 188, 188a, drops below the set point (985 psi in theexample previously given) of unloading relief valve 188 of module 38,poppet piston 190 of valve 188 closes against seat 186. Reloading valve188a closes when P drops below its set point, 500 psi in the examplepreviously given.

Poppet piston 148 of module 34 remains open as long as the fluid actingagainst it remains above the pressure required to overcome the bias ofspring 166 (in the example given, about 15 psi). As the pressure of thisfluid is determined by reloading valve 188a, and is equal to P -SOO psi(the set point of valve 162a), poppet piston 148 starts to close when Pdrops to about 515 psi and completely closes by the time P reaches 500psi, at which time reloading valve 188a closes and the pressure inconduits 134, 178a drops to zero.

The closure of poppet piston 148 cuts off flow through module 34 topoppet spool 98 of module 32, the pressure of fluid acting on smallerdiameter end 102 of spool 98 drops to zero, and spring 112 closes spoollarger diameter end 104 over seat 96.

Flow from source 16 through valve to reservoir 26 is now cut off, thepressure in conduit 38 immediately builds up thereby opening check valve60, and the cycle repeats.

It will be noted that the load pressure at which fluid from source 16 isdiverted to reservoir 26 depends on the setting of adjustable unloadingrelief valve 188; and the pressure at which inlet flow is again directedthrough valve 10 to load 22 is controlled by the set point of adjustablereloading relief valve 188a. Both the unloading and reloading points areadjustable, and they may be set as close as about l5 psi of each other,if desired. The instability range of unloading valve 10, determined bythe pressure exerted by spring 112 which is required to overcome theforce tending to hold poppet spool open (caused by flow from conduits 88past lip 96 to conduit 78) when the pressure of fluid from conduit 90 onend 103 of the spool has been dropped to zero, is less than 10 psi.

Other embodiments within the scope of the following claims will occur tothose skilled in the art.

What is claimed is:

1. A fluid control circuit comprising:

a controller including a control element movable between first andsecond positions and operative for generating a fluid output signaldependent on the position of said element;

a'first pressure sensor connected to said controller and comprising afirst valve body having a first pressure relief valve disposed thereinfor controlling flow from an inlet thereof to an outlet thereof; and,

a second pressure sensor connected to said controller and comprising asecond valve body having a second pressure relief valve disposed thereinfor controlling fluid from an inlet thereof to an outlet thereof,

each of said inlets being adapted for connection to a source of pilotfluid to sense the pressure thereof,

said controller including a third valve body having said control elementdisposed therein, a first control inlet connected to said outlet of saidfirst valve body, a second control inlet connected to said outlet ofsaid second valve body, and an outlet,

said first sensor being operable for causing said control element toassume its second position when the sensed pilot fluid pressure exceedsa predetermined upper level, and 5 said second sensor being operable forcausing said control element to assume its first position when saidsensed pilot fluid pressure is less than a predetermined lower level.

2. The fluid control circuit of claim 1 wherein at least one of saidsensors is adjustable for varying the respective one of said first andsecond predetermined levels.

3. The fluid control circuit of claim 1 wherein said control elementprevents flow through said third valve body from said one inlet to saidoutlet thereof in said first position thereof, permits said flow fromsaid one inlet to said outlet in said second position thereof, andpermits flow through said third valve body from said another inlet tosaid outlet in both said first and second position thereof.

4. The fluid control circuit of claim 3 wherein said controller isarranged such that when said control element is in said first positionthereof flow of fluid from said second sensor to said controller isineffective to cause movement of said control element to said secondposition thereof.

5. The fluid control circuit of claim 4 wherein said controller isarranged such that flow of fluid from said first sensor to saidcontroller is effective to cause said control element to assume saidsecond position thereof.

6. The fluid control circuit of claim 4 wherein said controller isarranged such that when said control ele ment is in said second positionthereof flow of fluid from said second sensor to said controller iseffective to maintain said control element in said second positionthereof.

7. The fluid control circuit of claim 3 wherein said control element isbiased towards said first position thereof.

8. The fluid control circuit of claim 3 including a fourth valve bodyhaving a control inlet connected to said third valve body outlet, a mainflow inlet, an outlet, and a bypass valve disposed therein forcontrolling flow therethrough between said main flow inlet and saidoutlet and responsive to flow from said control inlet.

9. Thefluid control circuit of claim 3 wherein said bypass valvecomprises a poppet spool movable between a closed position preventingsaid flow therethrough and an open position permitting said flowtherethrough and is biased towards said closed position, one end of saidspool defining a control surface arranged such that fluid from saidcontroller applied to said control surface tends to move said spool intoits open position.

10. The fluid control circuit of claim 1 including a plurality ofinterconnected valving modules;

a first one of said modules comprising said first sensor;

a second one of said modules comprising said second sensor;

a third one of said modules comprising said controller, and beingarranged such that said control element prevents flow from said secondcontrol inlet to said outlet in said first position thereof, permitssaid flow from said second control inlet to said outlet in said secondposition thereof, and permits flow from said first control inlet to saidoutlet in said first and second positions thereof; and,

a fourth one of said modules including a control inlet connected to saidoutlet of said third one module, a main flow inlet and a main flowoutlet, and having a bypass flow element disposed therein arranged forcontrolling flow between said main flow inlet and said main flow outlet,

said controller being further arranged such that flow from said secondsensor to said controller when said control element is in said firstposition is ineffective to cause movement of said control element tosaid second position thereof, said flow from said second sensor whensaid control element is in said second position thereof is effective tomaintain said control element in said second position thereof, and flowfrom said first sensor to said controller is effective to cause saidcontrol element to assume said second position thereof, and

said bypass flow element comprising a poppet spool movable between aclosed position preventing flow from said main inlet to said main flowoutlet and an open position permitting said flow from said main inlet tosaid main outlet, said spool being biased towards its closed positionand one end of said spool defining a control surface arranged such thatfluid from said controller to said control inlet applied to said controlsurface tends to move said spool into its open position.

11. The fluid control circuit of claim 10 wherein a fifth one of saidmodules includes a main flow inlet conduit having an inlet adapted to beconnected to a source of fluid, an outlet adapted to be connected to aload, a check valve disposed therein and biased towards its closedpposition for controlling flow therethrough,

a first bypass flow conduit connecting the upstream side of said checkvalve in said main flow conduit to said main flow inlet of said fourthone module,

a second bypass flow conduit extending from a bypass flow outlet of saidfifth one module to said main flow outlet of said fourth one module, and

a pilot pressure conduit extending from the downstream side of saidcheck valve in said main flow conduit to a pilot outlet,

and wherein said circuitry further includes means connecting the inletsof each of said first one and second one modules to said pilot outlet ofsaid fifth one module.

12. A fluid control circuit comprising:

a controller including a control element movable between first andsecond positions and operative for generating a fluid output signaldependent on the position of said element;

a first pressure sensor connected to said controller and adapted forconnection to a source of pilot fluid to sense the pressure thereof;and,

a second pressure sensor connected to said controller and adapted forconnection to said source to sense the pressure of said pilot fluid,

said first sensor being operable for causing said control element toassume its second position when the sensed pilot fluid pressure exceedsa predetermined upper level, and for passing fluid from said pilotsource to said controller along a first path when said sensed fluidpressure exceeds said upper level and for preventing flow along saidfirst path when said sensed fluid pressure is less than said upperlevel, and

said second sensor being operable for causing said control element toassume its first position when said sensed pilot fluid pressure is lessthan a predetermined lower level, and for passing flow from said sourceto said controller along a second path when said sensed fluid pressureexceeds said lower level and for preventing flow along said second pathwhen said sensed fluid pressure is less than said lower level.

13. The fluid control circuit of claim 12 wherein when said controlelement is in said first position said controller and sensors areoperative to cause said control element to remain in said first positionwhen said sensed pressure is intermediate said upper and lower levels.

14. The fluid control circuit of claim 12 wherein when said controlelement is in said second position said controller and sensors areoperative to cause said control element to remain in said secondposition when said sensed pressure is intermediate said upper and lowerlevels.

15. The fluid control circuit of claim 12 wherein said control elementis biased towards said first position thereof and is operative toprevent flow of fluid through said controller from said second sensor insaid first position thereof.

16. The fluid control circuit of claim 12 including a valve body havinga bypass flow valve disposed therein, a control inlet connected to saidcontroller, a main inlet for connection to a fluid source, and anoutlet, said bypass flow valve being movable between a closed positionpreventing flow from said main inlet through said valve body to saidoutlet andan open position permitting said flow through said valve body,and said controller being operative to move said bypass flow valve intothe open position thereof in response to movement of said controlelement into said second position thereof.

17. The fluid control circuit of claim 16 wherein said bypass flow valveis biased towards said closed position thereof.

18. The fluid control circuit of claim 16 wherein each of said sensorsis adjustable for varying the respective one of said upper and lowerpredetermined levels.

1. A fluid control circuit comprising: a controller including a controlelement movable between first and second positions and operative forgenerating a fluid output signal dependent on the position of saidelement; a first pressure sensor connected to said controller andcomprising a first valve body having a first pressure relief valvedisposed therein for controlling flow from an inlet thereof to an outletthereof; and, a second pressure sensor connected to said controller andcomprising a second valve body having a second pressure relief valvedisposed therein for controlling fluid from an inlet thereof to anoutlet thereof, each of said inlets being adapted for connection to asource of pilot fluid to sense the pressure thereof, said controllerincluding a third valve body having said control element disposedtherein, a first control inlet connected to said outlet of said firstvalve body, a second control inlet connected to said outlet of saidsecond valve body, and an outlet, said first sensor being operable forcausing said control element to assume its second position when thesensed pilot fluid pressure exceeds a predetermined upper level, andsaid second sensor being operable for causing said control element toassume its first position when said sensed pilot fluid pressure is lessthan a predetermined lower level.
 2. The fluid control circuit of claim1 wherein at least one of said sensors is adjustable for varying therespective one of said first and second predetermined levels.
 3. Thefluid control circuit of claim 1 wherein said control element preventsflow through said third valve body from said one inlet to said outletthereof in said first position thereof, permits said flow from said oneinlet to said outlet in said second position thereof, and permits flowthrough said third valve body from said another inlet to said outlet inboth said first and second position thereof.
 4. The fluid controlcircuit of claim 3 wherein said controller is arranged such that whensaid control element is in said first position thereof flow of fluidfrom said second sensor to said controller is ineffective to causemovement of said control element to said second position thereof.
 5. Thefluid control circuit of claim 4 wherein said controller is arrangedsuch that flow of fluid from said first sensor to said controller iseffective to cause said control element to assume said second positionthereof.
 6. The fluid control circuit of claim 4 wherein said controlleris arranged such that when said control element is in said secondposition thereof flow of fluid from said second sensor to saidcontroller is effective to maintain said control element in said secondposition thereof.
 7. The fluid control circuit of claim 3 wherein saidcontrol element is biased towards said first position thereof.
 8. Thefluid control circuit of claim 3 including a fourth valve body having acontrol inlet connected to said third valve body outlet, a main flowinlet, an outlet, and a bypass valve disposed therein for controllingflow therethrough between said main flow inlet and said outlet andresponsive to flow from said control inlet.
 9. The fluid control circuitof claim 3 wherein said bypass valve comprises a poppet spool movablebetween a closed position preventing said flow therethrough and an openposition permitting said flow therethrough and is biased towards saidclosed position, one end of said spool defining a control surfacearranged such that fluid from said controller applied to said controlsurface tends to move said spool into its open position.
 10. The fluidcontrol circuit of claim 1 including a plurality of interconnectedvalving modules; a first one of said modules comprising said firstsensor; a second one of said modules comprising said second sensor; athird one of said modules comprising said controller, and being arrangedsuch that said control element prevents flow from said second controlinlet to said outlet in said first position thereof, permits said flowfrom said second control inlet to said outlet in said second positionthereof, and permits flow from said first control inlet to said outletin said first and second positions thereof; and, a fourth one of saidmodules including a control inlet connected to said outlet of said thirdone module, a main flow inlet and a main flow outlet, and having abypass flow element disposed therein arranged for controlling flowbetween said main flow inlet and said main flow outlet, said controllerbeing further arranged such that flow from said second sensor to saidcontroller when said control element is in said first position isineffective to cause movement of said control element to said secondposition thereof, said flow from said second sensor when said controlelement is in said second position thereof is effective to maintain saidcontrol element in said second position thereof, and flow from saidfirst sensor to said controller is effective to cause said controlelement to assume said second position thereof, and said bypass flowelement comprising a poppet spool movable between a closed positionpreventing flow from said main inlet to said main flow outlet and anopen position permitting said flow from said main inlet to said mainoutlet, said spool being biased towards its closed position and one endof said spool defining a control surface arranged such that fluid fromsaid controller to said control inlet applied to said control surfacetends to move said spool into its open position.
 11. The fluid controlcircuit of claim 10 wherein a fifth one of said modules includes a mainflow inlet conduit having an inlet adapted to be connected to a sourceof fluid, an outlet adapted to be connected to a load, a check valvedisposed therein and biased towards its closed pposition for controllingflow therethrough, a first bypass flow conduit connecting the upstreamside of said check valve in said main flow conduit to said main flowinlet of said fourth one module, a second bypass flow conduit extendingfrom a bypass flow outlet of said fifth one module to said main flowoutlet of said fourth one module, and a pilot pressure conduit extendingfrom the downstream side of said check valve in said main flow conduitto a pilot outlet, and wherein said circuitry further includes meansconnecting the inlets of each of said first one and second one modulesto saId pilot outlet of said fifth one module.
 12. A fluid controlcircuit comprising: a controller including a control element movablebetween first and second positions and operative for generating a fluidoutput signal dependent on the position of said element; a firstpressure sensor connected to said controller and adapted for connectionto a source of pilot fluid to sense the pressure thereof; and, a secondpressure sensor connected to said controller and adapted for connectionto said source to sense the pressure of said pilot fluid, said firstsensor being operable for causing said control element to assume itssecond position when the sensed pilot fluid pressure exceeds apredetermined upper level, and for passing fluid from said pilot sourceto said controller along a first path when said sensed fluid pressureexceeds said upper level and for preventing flow along said first pathwhen said sensed fluid pressure is less than said upper level, and saidsecond sensor being operable for causing said control element to assumeits first position when said sensed pilot fluid pressure is less than apredetermined lower level, and for passing flow from said source to saidcontroller along a second path when said sensed fluid pressure exceedssaid lower level and for preventing flow along said second path whensaid sensed fluid pressure is less than said lower level.
 13. The fluidcontrol circuit of claim 12 wherein when said control element is in saidfirst position said controller and sensors are operative to cause saidcontrol element to remain in said first position when said sensedpressure is intermediate said upper and lower levels.
 14. The fluidcontrol circuit of claim 12 wherein when said control element is in saidsecond position said controller and sensors are operative to cause saidcontrol element to remain in said second position when said sensedpressure is intermediate said upper and lower levels.
 15. The fluidcontrol circuit of claim 12 wherein said control element is biasedtowards said first position thereof and is operative to prevent flow offluid through said controller from said second sensor in said firstposition thereof.
 16. The fluid control circuit of claim 12 including avalve body having a bypass flow valve disposed therein, a control inletconnected to said controller, a main inlet for connection to a fluidsource, and an outlet, said bypass flow valve being movable between aclosed position preventing flow from said main inlet through said valvebody to said outlet and an open position permitting said flow throughsaid valve body, and said controller being operative to move said bypassflow valve into the open position thereof in response to movement ofsaid control element into said second position thereof.
 17. The fluidcontrol circuit of claim 16 wherein said bypass flow valve is biasedtowards said closed position thereof.
 18. The fluid control circuit ofclaim 16 wherein each of said sensors is adjustable for varying therespective one of said upper and lower predetermined levels.